Hybrid electric vehicle powertrain systems frequently utilize a high voltage energy storage system, such as a battery pack, to provide electric energy to an electric drive motor to produce the necessary output torque to crank the combustion engine. Under most conditions, the battery pack has adequate energy capability and capacity to crank and start the combustion engine. However, under certain circumstances the capacity of the battery may not be sufficient to start the engine. For example, in extremely cold ambient temperature conditions the state of the fuel and its ability to be delivered to the engine, high frictional losses and other factors may make the combustion engine difficult to start. Under such conditions, more battery power may be required to crank the engine. In addition, lack of fuel or other factors either internal or external to the powertrain system may make it difficult or impossible for the engine to start, thereby leading to the use of excessive amounts of battery power to crank the engine.
Repeated cranking attempts due to cold ambient conditions, lack of fuel, or other factors can result in depletion of and potential degradation of the high voltage energy storage system. The primary causal factor associated with battery degradation under discharge conditions is extremely low battery voltage.
Typical undervoltage battery protection algorithms establish a low voltage threshold or limit. When the actual system output voltage falls below the threshold, further discharge is disabled to protect the battery system. Repeated discharge attempts, such as those caused by cranking an engine that does not start, may result in the battery voltage reaching the low voltage threshold, whereupon the system will not permit additional discharge of the battery and no additional cranking of the engine is possible. This may result in an inoperable vehicle without providing warning to the operator that future start attempts would be inhibited.
Therefore, it is desirable to provide battery undervoltage protection which limits cranking in an undervoltage condition while also reserving sufficient battery energy for a number of crank attempts. It is also desirable to provide an indication to an operator of the number of remaining crank attempts or that future crank attempts may be restricted.